<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>浮动float</title>
    <style>
        .mainDiv{
            width:90%;
            hright:300px;
        }
        .first {
            background-color:aqua;
            height:100px;
            width:20%;
            float:left;
        }
        .second {
            background-color:red;
            width:20%;
            height:100px;
            float:left;
        }
        .third {
            background-color:green;
            height:100px;
            width:20%;
            float:left;
        }
    </style>
</head>
<body>
<div class="mainDiv">
    <div class="first">学术活动</div>
    <div class="second">22222</div>
    <div class="third">33333</div>
</div>
</body>
</html>